1. Field of the Invention
The present invention relates generally to spacecraft and, more particularly, to spinning spacecraft.
2. Description of the Related Art
The perspective view 20 of FIG. 1 illustrates spacecraft which are orbiting the Earth 22 in an orbital plane 23 for various uses (e.g., as communication satellites). Conventional configurations of these spacecraft include body-stabilized spacecraft and spin-stabilized spacecraft which are described in various spacecraft references (e.g., Morgan, Walter L., et al., Communications Satellite Handbook, John Wiley & Sons, New York, 1989, pp. 547-554 and 636-641).
In particular, FIG. 1 includes a body-stabilized satellite 24 which is fixed with respect to a coordinate system 26 that has an origin at the satellite's center of mass. The coordinate system 26 has a yaw axis 27 which is directed from the origin towards a point on the Earth 20. A pitch axis 28 is directed from the origin and is orthogonal to the satellite's orbital plane 23. A roll axis 29 is orthogonal to the other two axes and is aligned with the satellite's velocity vector. As the body-stabilized satellite 24 orbits the Earth 22, its yaw axis 27 typically rotates so that it is constantly directed at the Earth's center of mass. Body-stabilized spacecraft generally have momentum wheels (or reaction wheels) and multiple thrusters which are periodically operated to control the spacecraft's attitude with respect to its coordinate system.
As shown in FIG. 1, one or more solar cell arrays 30 are typically extended from the spacecraft on rotatable booms 32 which are generally coaxial with the pitch axis 28. The booms 32 are then rotated so as to keep the solar cell arrays directed towards the Sun as the spacecraft 24 orbits the Earth 22.
The current and power output of the solar cells varies as the cosine of the angle between a normal to the plane of the array and the Sun line (an imaginary line between the Sun and the spacecraft). When the Sun is at spring or fall equinox, the booms can be rotated to place the plane of the solar cell arrays 30 orthogonal to the Sun line. In this orientation, the solar cell arrays receive the maximum energy from the Sun and generate their maximum current output.
Because the ecliptic is rotated from Earth's equatorial plane by .about.23.5 degrees, the plane of the solar cell arrays 30 describes the same angle with the Sun line when the Sun is at summer and winter solstice. The current from the solar cells is then approximately 92% (.about.cosine 23.5 degrees) of the maximum output. This drop in power level can be avoided by rotating the entire spacecraft about its yaw axis 27 so that the solar cell arrays 30 are Sun-steered (maintained in an orthogonal relationship with the Sun line).
In contrast to the body-stabilized satellite 20, all or a significant portion of a spin-stabilized spacecraft rotates at a predetermined rate (e.g., once a second). An exemplary spin-stabilized spacecraft is the spun drum 40 which is also shown orbiting the Earth 22 in FIG. 1. The spun drum has a generally cylindrical shape and rotates about a rotational axis 42 which is its axis of maximum moment of inertia. The spinning is indicated by a rotational vector 43. Because this axis corresponds to a least energy state, the spun drum 40 is passively stable.
The outer surface of the spun drum 40 is covered by a cylindrical array 44 of solar cells. Because of the cylindrical shape, one half of the solar cells are hidden from the Sun at any instant of time and those of the other one half curve away from the Sun. Accordingly, the cylindrical array 44 of the spun drum 40 requires .about. .pi. times as much array area as the flat array 30 of the body-stabilized spacecraft 24 to generate the same current. In addition, the power output of the solar cell array 44 drops during summer and winter solstice for the same reason described above for the body-stabilized spacecraft 24.
The spun drum 40 is sometimes modified to have a portion or shelf 46 which is despun. Equipment (e.g., an antenna 48) can be carried on the shelf 46 and directed at a coverage area on the Earth 22.
Another exemplary spacecraft is the spun deployed-fixed spacecraft 60 of FIG. 1. This spacecraft rotates about a spin axis 62 and the spacecraft is oriented with the spin axis in the orbital plane 23. The spinning is indicated by a rotational vector 63. A plurality of planar panels 64 are canted back from the spin axis 62 and a solar cell array 66 is carried on each side of each panel. With its double-sided solar cell arrays, the spun deployed-fixed spacecraft 60 can maintain a more constant level of generated power than the spun drum 40. However, this advantage is gained with a significant penalty of additional weight and cost due to the large number of solar cells on both sides of the deployed panels 64.
Spin-stabilized and spinning spacecraft are typically less expensive than body-stabilized spacecraft because they are generally less complex (e.g., they require less hardware in the form of momentum wheels and thrusters). Spinning also assists propellant distribution by utilizing centrifugal force. Potentially, this elminates the need for pressurant tanks. Consequently, this class of spacecraft is an attractive choice for many spacecraft missions.
As discussed above, however, the solar cell arrays of spin-stabilized and spinning spacecraft typically cannot generate a constant power output throughout an orbit and/or require an excessive number of solar cells. A nonconstant power output requires that the spacecraft have additional batteries and more complex power processing systems. Additional solar cells add cost, weight and stowed volume. These cost, weight and volume penalties have typically reduced the payload and revenues of spin-stabilized and spinning spacecraft.